UV absorbing borosilicate glass compositions for electric arc discharge lamps such as high intensity discharge (HID) lamps, typically contain either lead and arsenic oxides or lead and cerium oxides. One conventional lead and arsenic containing borosilicate glass is SG772. SG772 has physical properties which enables it be used for both outer envelope and lamp stem applications. However, both lead and arsenic are toxic materials and it would be very advantageous to be able to manufacture acceptable glasses without using these materials. Arsenic oxide is generally employed in glass compositions as a fining agent for glasses which are difficult to fine (i.e., removal of bubbles). Ceria (&gt;0.15 wt. %) has been used as an acceptable substitute for arsenic oxide for fining glasses, and providing UV absorption. However, ceria-containing borosilicate glasses will solarize under UV irradiation when lead oxide is not in the composition.
Solarization produces light absorbing color centers that darken the outer envelope and seriously reduce light output.
Lead and arsenic free borosilicate glass compositions have been developed to exclude these toxic components. One such lead and arsenic free borosilicate glass (designated SG773) is described in commonly assigned, copending U.S. application Ser. No. 09/085,989, filed May 5, 1998 which is incorporated herein by reference. SG773 glass is typically manufactured in a glass melter equipped with a bubbler system. Bubblers improve convection, and control the flow of glass through the melter. Large bubbles are introduced at the bottom of the melter which rise to the surface. The convective mixing which develops with bubbling brings cold bottom glass up to the melt surface for additional melting. This increases the average glass temperature and melting rate. Without bubbling, the radiative heat transfer in SG773 glass must be enhanced to make up for the decrease in convective mixing. Thus, it would be an advantage to make this glass more suitable for use in glass melters which are not equipped with bubblers.